In 3GPP Long-Term Evolution (LTE) networks, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of base stations, e.g., evolved Node-Bs (eNBs) communicating with a plurality of mobile stations referred as user equipments (UEs). Orthogonal Frequency Division Multiple Access (OFDMA) has been selected for LTE downlink (DL) radio access scheme due to its robustness to multipath fading, higher spectral efficiency, and bandwidth scalability. Multiple access in the downlink is achieved by assigning different sub-bands (i.e., groups of subcarriers, denoted as resource blocks (RBs)) of the system bandwidth to individual users based on their existing channel condition. In LTE networks, Physical Downlink Control Channel (PDCCH) is used for downlink (DL) scheduling or uplink (UL) scheduling of Physical Downlink Shared Channel (PDSCH) or Physical Uplink Shared Channel (PUSCH) transmission. The DL/UL scheduling information carried by PDCCH is referred to as downlink control information (DCI).
The DCI format is a predefined format in which the downlink control information is formed and transmitted in PDCCH. The DCI format gives the UE details such as number of resource blocks, resource allocation type, modulation scheme, transport block, redundancy version, coding rate etc. Different DCI formats are defined in LTE to support different transmission schemes. For example, DCIs can have six formats: one format for UL scheduling, two formats for non-MIMO DL scheduling, one format for MIMO DL scheduling, and two formats for UL power control. When a new feature is introduced, new DCI format may be defined and the DCI size may change accordingly. For example, for UE with one Hybrid automatic repeat request (HARQ) process there is no field in DCI to indicate HARQ process number. However, to support more than one HARQ processes, the field in DCI to indicate HARQ process number is needed such that the UE knows which HARQ process the scheduled grant belongs to. In another example, a UE with larger bandwidth may need more bits for resource allocation, which results in a different DCI format/size.
In LTE, single-step DL/UL data scheduling is adopted. Each UE needs to blindly decode PDCCH over specified search spaces within a single step. This has a few disadvantages. First, it has poor DCI forward compatibility because the size of DCI is specified. When a new feature is introduced with DCI changes, it introduces large specification impact on DCI size determination and UE blind detection behavior. Second, it has poor hardware flexibility because the DCI size is usually hardware-coded for fast DCI decoding. When a new feature is introduced with DCI changes, it introduces large hardware impact. A solution is sought to enhance DCI forward compatibility and hardware flexibility.